1. Field of the Invention
The present invention generally relates to the field of high-speed electronic transistor devices, and more specifically to an NPN heterojunction bipolar transistor (HBT) including an antimonide base formed on a semi-insulating indium phosphide (InP) substrate.
2. Description of the Related Art
HBTs provide substantial advantages over conventional homojunction bipolar transistors by enabling energy-gap variations in addition to electric fields as forces acting on electrons and holes. In an HBT, the emitter is designed to have a wider bandgap than the base, creating an energy barrier in the valence band at the emitter-base junction which inhibits unwanted flow of holes from the base to the emitter and substantially increases the emitter injection efficiency, current gain and operating frequency.
The wide-bandgap emitter enables very high base doping, allowing low base resistance to be obtained even with small base widths. The emitter doping can be reduced to moderate levels, allowing a reduction in base-emitter capacitance.
The advantages of HBTs have been extensively demonstrated for high-speed aluminum-gallium-arsenide/gallium-arsenide (AlGaAs/GaAs) HBTs. Moreover, alternative material systems for fabricating HBTs, including indium phosphide/indium-gallium-arsenide (InP/InGaAs) and aluminum-indium-arsenide/indium-gallium-arsenide(AlInAs/InGaAs) offer even higher performance as described in an article entitled "AlInAs/GaInAs HBT Technology" by J. Jensen et al, in IEEE Journal of Solid-State Circuits, Vol. 26, No. 3, March 1991, pp. 415-421.
However, HBTs fabricated using the latter material systems have maximum frequencies of oscillation (f.sub.max) which are lower than their respective high cutoff frequencies (f.sub.T) due to relatively high values of the product of base resistance and collector capacitance. This limits the potential speed and output power of circuits incorporating these HBTs because, as a desired design parameter, f.sub.max should be approximately twice f.sub.T.
Moreover, the base layer of InP/InGaAs and AlInAs/InGaAs/InGaAs HBTs is typically doped with beryllium (Be) as a P-type dopant. Be is highly diffusive, and migrates rapidly from the base layer into the emitter layer during growth and even during device operation, causing the P-N junction to be displaced from the emitter-base junction into the emitter layer. As a consequence, an energy barrier to electron flow is created and the barrier to hole flow is reduced, thus reducing the current gain.
Therefore, there is a need for an HBT device which overcomes these problems that has enhanced performance characteristics, such as high f.sub.max than the conventional HBTs, and is doped with Be or other P-type dopant without conventional notorious effects.